In nuclear medicine, radiopharmaceutical are commonly injected into the subject's blood stream for imaging the circulatory system or for imaging specific organs which absorb the injected radiopharmaceutical. Depending on the type of radiopharmaceutical injected, either SPECT or PET imaging is typically used to obtain a final image.
In single photon emission computed tomography imaging, a single photon emitting radiopharmaceutical such as .sup.201 T1 is introduced into a subject or object. A detector is placed closely adjacent to a surface of the subject to monitor radiation characteristic of the radiopharmaceutical's decay. The detector includes a collimator which allows only photons traveling along a relatively limited angle of incidence to reach the detector. An image of the subject is reconstructed utilizing the information obtained by the detected photons. While SPECT imaging may be accomplished using a gamma camera having only one detector, gamma cameras having two or more detectors may also be used.
Positron emission tomography is a branch of nuclear medicine in which a positron-emitting radiopharmaceutical such as .sup.18 F-Fluorodeoxyglucose (FDG) is introduced into the body of a patient. Each emitted positron reacts with an electron in what is known as an annihilation event, thereby generating a pair of 511 keV gamma rays. The gamma rays are emitted in directions approximately 180.degree. apart, i.e. in opposite directions. A pair of detectors registers the position and energy of the respective gamma rays, thereby providing information as to the position of the annihilation event and hence the positron source. Coincidence circuitry is used to determine if a pair of gamma rays is detected substantially simultaneously, e.g., in coincidence. Because the gamma rays travel in opposite directions, the positron annihilation is said to have occurred along a line of coincidence connecting the detected gamma rays. A number of such events are collected and used to reconstruct an image. While an imaging apparatus having at least two detectors is required for PET imaging, additional detectors may also be used.
A drawback to both SPECT and PET imaging technique is that the subject or object being imaged may not be completely homogeneous in terms of radiation attenuation or scatter. For example, a human patient includes many different tissue and bone types which absorb or scatter radiation from the radiopharmaceutical to different degrees. Thus, both SPECT and PET images can be made more accurate if they are corrected for the radiation lost to scattering or attenuation along each path through the human.
Accordingly, it is known to measure the actual attenuation coefficients of body tissues by placing a transmission source of gamma radiation such as a line source on one side of the body and measuring the transmission of the gamma radiation through the body. More specifically, gamma radiation originating from the line source and having passed through the body is detected by one of the gamma camera detectors and used to correct for attenuation and possibly scatter in an image reproduced from the detected gamma rays of the radiopharmaceutical. Unfortunately, existing line sources, and existing gamma camera systems that use them, suffer from certain disadvantages.
For instance, as disclosed in U.S. Pat. No. 5,479,021, which is assigned to Picker International, Inc, a fan beam radiation line source is mounted to a rotating gantry between two detectors and opposite a third. A drawback to this mounting arrangement is that it is not applicable to opposed, two detector head system. Further, such mounting arrangement would not be well suited for systems in which detectors move relative to one another since a detector currently positioned opposite the line source may move from that position.
One technique for utilizing a line source in a system having opposed detectors is to mount the line source at the side of one of the opposed detectors. The line source may then direct a fan beam of radiation to the opposed detector. Such a configuration is shown in one embodiment of U.S. Pat. No. 5,210,421 assigned to Picker International, Inc. A drawback to this approach is that the collimator of the opposed detector must be modified to allow detection of the transmitted radiation from the line source. More particularly, the collimator of the opposed detector would need to be configured to receive the fan beam of radiation. In SPECT imaging, such a collimator configuration typically results in a deleterious effect on the detector's field of view and artifacts from data truncation. Further, such mounting configuration is not well suited for systems in which detector move relative to one another.
In U.S. Pat. No. 5,552,606 assigned to ADAC Laboratories, Inc., there is described yet another technique for utilizing a line source for attenuation correction. In the '606 patent, a line source is shown movably mounted to a rail opposite a detector so as to allow the line source to scan a parallel beam of radiation across the face of the opposing detector. Although the line source configuration of the '606 patent does not require that the opposing detector to have a collimator capable of receiving a fan beam of radiation, the line source assembly does necessitate the use of a complex mechanical arrangement to moveably support the line source and track its linear position. Further, the arrangement of the line source in the '606 patent does not allow for detector heads to be arranged opposite one another and is not suitable for use in systems in which detectors move relative to one another.
Still another technique for utilizing a line source in a system having opposed detectors is described in pending U.S. patent application Ser. No. 08/654,542, filed on May 29, 1996 (U.S. Pat. No. 5,834,780) and assigned to Picker International, Inc. In this application, there is described a scanning line source which is movably affixed to a detector face. By moving the line source across the detector face, a parallel beam of radiation may be directed to the opposing detector in a plane substantially orthogonal to its face. Thus, detector heads may be positioned opposite one another while still allowing parallel beam collimators (as opposed to less desirable fan beam collimators) to be used. While the application Ser. No. 08/654,542 provides clear advantages over other existing technologies, it requires the use of a mechanical mounting assemblies to affix the line source to a detector. Also, such a configuration is not well suited for use in systems having detectors which move relative one another.
While line sources are utilized in both SPECT and PET imaging, it will be appreciated that the line sources used in one are not compatible for use in the other. More particularly, line sources used in SPECT typically include a low energy isotope such as Gd-153 (100 keV), Tc-99m (140 keV), or Am-241 (60 keV). By comparison, line sources used in PET typically include a high energy isotope such as Ge-68 (511 keV-coincidence) or Cs-137 (622 keV--singles). If the low energy isotopes used in SPECT were replaced with the high energy isotopes used in PET several difficulties would arise. For one, because the radiopharmaceutical injected into a subject in SPECT is typically of low energy, the introduction of high energy radiation from a line source in such a system would require that a detector be capable of reliably detecting radiation in both the low energy and high energy ranges. Similarly, if a low energy isotope from a SPECT system were placed into a line source of a PET system which normally utilize only high energy radiopharmaceutical, the detector would again need to be able to handle a large range of energies. Unfortunately, many detectors are not able to reliably detect and reproduce images across such a large dynamic range of energies as is necessary to produce high-quality attenuation maps. Further, introduction of a high energy isotope to an otherwise low energy SPECT system would result in a significant increase of contamination in the energy window of the low energy radiopharmaceutical. More particularly, image quality is adversely affected since a significant amount of scatter from the radiation of the high energy isotope falls into the energy range of the low energy radiopharmaceutical thereby making it difficult for the detector to distinguish between radiation received from the subject and radiation received from the line source. Similarly, if a low energy isotope were introduced to a PET system having high energy radiopharmaceutical, scatter from the high energy radiopharmaceutical would significantly reduce the ability to detect radiation from the low energy isotope. Thus, line sources are individually configured for use in either a SPECT or a PET system.
Another drawback to the many prior art line source techniques is that radiation emitted by the line source but not attenuated by the subject reaches the detector without substantial attenuation. This "shine by" radiation results in extraneous detector counts and can cause saturation of the detector, leading to inaccuracies in the image data.
The present invention contemplates a new and improved scanning line source which overcomes the above mentioned shortfalls and others.